Core
The core of a transformer is the area through which the magnetic field circulates between the primary and secondary windings. Depending on the purpose of the transformer, it can have various shapes and be made of different materials.
The core is usually made up of several sheets of metal (usually ferromagnetic material) that are stacked next to each other, without welding, similar to the pages of a book. The function of the core is to keep the magnetic flux confined within it and prevent it from flowing through the air, favoring losses in the core and reducing efficiency. The laminated configuration of the laminated core is done to reduce eddy currents and consequently reduce energy losses in the core.
Some transformers do not have a core and are called coreless or air core transformers. An air core is essentially a core with no hysteresis or eddy current losses. However, the leakage inductance is very high, making them inappropriate for power transmission. On the contrary, they have a very high bandwidth and are frequently used in radio communication applications. The concept of air core transformers also includes wireless charging systems and Tesla coils.
A winding is wound around each column. An upper yoke and a lower yoke close the magnetic circuit of the core.
In addition to the columns, two side cylinder heads without windings are arranged on the sides. They prevent dispersion flows.
The core consists of a ring (torus), usually made of artificial ferrite compounds, on which the primary and secondary are wound. They are more voluminous, but the magnetic flux is confined to the core, having very low dispersion fluxes and low losses due to eddy currents.
The core is formed by a grain-oriented iron sheet, wound on itself, always in the same direction, instead of the usual separate soft iron sheets. It has very low losses, but it is expensive. Grain-oriented iron sheet can also be used in oriented transformers (E-sheet), reducing their losses.
In high frequency applications, windings are used on a reel without a core or with a small cylinder of ferrite "Ferrite (ferromagnetic ceramic)") that is more or less introduced into the reel, to adjust its inductance.
Coils
The coils are generally copper wound on the core. Depending on the number of coils (turns) around a leg, a higher voltage will be induced. It is then played with the number of turns in the primary versus those in the secondary. In a three-phase transformer the number of turns of the primary and secondary should be the same for all phases.
The primary and secondary windings are usually wound inside each other. The reason is to reduce the leakage inductance as much as possible and make the most of the available magnetic core. An insulating layer is necessary between the windings, since both operate at different voltages. To avoid having too thick insulating layer thicknesses, it is most common to find the low-voltage winding wound on the core and the high-voltage winding wound on the low-voltage winding.
Other components
Depending on the size and purpose of the transformer, it may also have the following components.
The tap changer is an electromechanical device that can be manually operated to change the transformation ratio in a transformer. Typically, there are five steps or shots: one of them is neutral (represents the nominal transformation ratio) and the others alter the transformation ratio by plus or minus 2.5% and plus or minus 5% respectively. This helps, for example, to correct the voltage in the secondary if it falls too much on any bus in the system.
It is a mechanical device that equalizes the pressure increase in the transformer that can cause it to explode. However, there are several computers that explode despite having this device. There are the sudden pressure relay for transient pressures and the overpressure relay for more permanent pressures.
This accessory indicates the pressure difference between the gas trapped between the transformer oil and its tank and the atmospheric pressure. It is used exclusively with oil-sealed tank transformers, a type of transformer used in installations no larger than 7500 kVA.
The pressure in the gas trapped between the oil and the tank undergoes expansion or contraction due to volume variations in the insulating liquid, variations in ambient temperature or in the transformer's own load. High negative or positive pressures can indicate abnormal conditions in the transformers, such as gas leaks or elements entering the tank.
This accessory is normally equipped with alarms, which allow extreme values of positive or negative pressure to be detected.
Allowing extreme pressures in the gas between the oil and the tank can cause warping or rupture of the tank.
The alarms of this device are normally linked to other transformer protection devices such as relief valves.[3].
In transformers of a certain size and primary cooling in an oil bath, an expansion tank is used to absorb the increase in volume produced by the heating of the coolant.
In large oil-bath cooled transformers, a Buchholz relay is used to detect failures of the dielectric (insulating material between the windings). This type of failure generates an electric arc inside the transformer that breaks down the oil. As a result of this chemical decomposition, gases are generated that accumulate in the Buchholz relay, causing the oil level to drop. A switch connected to the oil level triggers the relevant protections and alarms.
With the advent of electronics and the widespread use of microprocessors, different devices are currently on the market that allow transformer protection.
There are devices that incorporate protection functions specially designed for transformers, and that additionally monitor changes in the electromechanical protections included in them; This monitoring occurs through digital contacts and its change from open to closed is sensed, resulting in the opening of the switches associated with the transformer, the blocking of its closure or alarms.